1. Technical Field
The invention relates generally to devices for emitting white light, and more specifically, to a device that can generate white light having one or more alterable and/or selectable properties.
2. Related Art
Traditionally, the general illumination market has comprised lighting devices such as incandescent bulbs and mercury-containing discharge tubes. In general, different lighting needs can be served by different bulbs. For example, general outdoor lighting can use lower cost bulbs that provide lower color rendering, while store lighting generally requires a high color rendering. However, most general illumination applications require some form of white light.
Recently, light emitting diodes (LEDs) have started to be used in high-power devices, and are no longer limited to uses such as small indicator lamps. Further, LEDs are generally more energy efficient than the lighting devices traditionally used in the general illumination market. As a result, LEDs are considered an attractive alternative to traditional general lighting devices, and are encroaching on applications in the general illumination market.
However, LEDs are inherently chromatic (e.g., non-white) sources of light. In general, two solutions have been used to generate white light using one or more LEDs. In one solution, a single chip partially converts blue light or completely converts near-ultraviolet light emitted by an LED to light emitted over a broad spectrum (e.g., white light) through the use of an ionic phosphor layer. For example, an LED can emit blue light, some of which is converted to yellow light by a phosphor layer and some of which escapes unchanged. As a result, the device emits white light that is a dichromatic combination of yellow-blue (YB) light. Alternatively, a multi-phosphor blend can be used to generate light in multiple chromatics such as tri-chromatic red-green-blue (RGB) light. Advanced phosphor-conversion LEDs that exploit multi-phosphor blends exhibit excellent color rendering properties and are available for a variety of color temperatures.
However, phosphor-conversion white LEDs also have some drawbacks. For example, energy is expended during the conversion process, making the device less efficient. Further, different temperature behavior and different rates of deterioration of the phosphors and the semiconductor chip result in an uncontrollable shift in chromaticity that makes the lifetime during which the device can generate acceptable white light shorter than the lifetime of the device itself.
In an alternative solution, light emitted from multiple LEDs having varying chromaticity can be mixed to generate white light. Despite a relatively narrow emission spectra of each LED, polychromatic color mixing devices that incorporate four or more primary sources can cover the entire visible spectrum and accurately render the colors of illuminated objects. For example, an optimized quadri-chromatic red-amber-green-blue (RAGB) device has been shown to feature high values of both the general and all the special color rendering indices.
Further, these devices can operate more efficiently than the phosphor-conversion white LEDs since there is no energy loss due to conversion. In addition, these devices allow for full color control, an ability to tradeoff between qualitative characteristics (e.g., efficiency) and quantitative characteristics (e.g., color rendering), incorporation of internal feedback for compensation of chromaticity variations due to aging, temperature, etc., and the like. However, further development of the multi-chip lighting devices is substantially hindered by the absence of efficient LEDs in the yellow-green region due to various issues with semiconductor band-structure and material.
As a result, a need exists for an improved method and device for generating white light. In particular, a need exists for a method and device that supplement a white light source, such as a phosphor-conversion white LED, with one or more supplemental LEDs. In this manner, one or more properties of the white light can be adjusted to extend the life of the device and/or based on an application for the device.